Iterable gensim

The strength of gensim is in processing large data, using lazy-loading streams.

I noticed your code puts all documents into RAM (as a plain `list`), so I changed it to use lazy iteration.

Also it seems you're not doing any preprocessing; LDA can be picky about that. So I added rudimentary preprocessing = lowercasing words & removing stopwords.

I tried to follow your non-PEP8 coding style, for visual consistency.